Random access channel for ofdm-mimo system

ABSTRACT

In orthogonal frequency division multiplexing (OFDM) multiple-input multiple-output (MIMO) systems, a wireless transmit/receive unit (WTRU) selects a random access channel (RACH) and a phase for a constant amplitude zero auto correlation (CAZAC) sequence for RACH transmission. The WTRU then transmits a RACH transmission to a Node B via the selected RACH. Once the RACH transmission is detected, the Node B sends an acknowledgement (ACK) to the WTRU over an ACK channel. The Node B may transmit the ACK on a shared channel. The WTRU may ramp up transmit power while the RACH transmission is transmitted, or steps up transmit power of a subsequent RACH transmission. The RACH transmission and data transmission may be either time multiplexed or frequency multiplexed. A plurality of RACHs may be defined and one of the defined RACHs may be selected randomly or based on predetermined criteria.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/224,463, filed Sep. 2, 2011, which is a continuation of U.S. patentapplication Ser. No. 11/746,363, filed May 9, 2007, which issued as U.S.Pat. No. 8,014,361 on Sep. 6, 2011, which claims the benefit of U.S.Provisional Application No. 60/798,970 filed May 9, 2006, which areincorporated by reference as if fully set forth.

FIELD OF INVENTION

The present invention relates to wireless communications. Moreparticularly, the present invention relates to a random access channel(RACH) in orthogonal frequency division multiplexing (OFDM)multiple-input multiple-output (MIMO) systems.

BACKGROUND

In a wireless communication system, a RACH is used by a user terminal toestablish a link with a base station for data transmission. The accessvia the RACH should not create undue interference on other communicationlinks in a cell and should allow differentiation for a large group ofusers. The channel that the user terminal uses to establish the initiallink with the base station is an RACH. The design of an RACH should meetsuch requirements as allowing the base station to detect access attemptseasily, allow enough capacity for a large number of terminals to accessthe base station without undue congestion, and allow differentiation forgroups of users since it is desirable for the base station todifferentiate access attempts.

SUMMARY

The present invention relates to an RACH in OFDM MIMO systems. Awireless transmit/receive unit (WTRU) selects an RACH and a phase for aconstant amplitude zero auto correlation (CAZAC) sequence for RACHtransmission. The WTRU then transmits a RACH transmission to a Node Bvia the selected RACH. Once the RACH transmission is detected, the NodeB sends an acknowledgement (ACK) to the WTRU over an ACK channel. TheNode B may transmit the ACK on a shared channel. The WTRU may ramp uptransmit power while the RACH transmission is transmitted, or steps uptransmit power of a subsequent RACH transmission. The RACH transmissionand data transmission may be either time multiplexed or frequencymultiplexed. A plurality of RACHs may be defined and one of the definedRACHs may be selected randomly or based on predetermined criteria. TheRACH transmission may be transmitted using one of space-frequency blockcoding (SFBC), space time block coding (STBC), and beam forming.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the invention may be had from thefollowing description of a preferred embodiment, given by way of exampleand to be understood in conjunction with the accompanying drawingswherein:

FIG. 1 shows a schematic flow of uplink data transmission in an OFDMsystem;

FIG. 2 shows sub-carrier mapping for an RACH in a localized transmissionoption;

FIG. 3 shows sub-carrier mapping for an RACH in a distributedtransmission option;

FIG. 4 shows a frame structure of a RACH in accordance with the presentinvention;

FIG. 5 shows multiple data frames with slots for RACH transmissionin-between them;

FIG. 6 shows multiple RACHs that are defined by different sets ofsub-carriers;

FIG. 7 shows a wireless communication system in accordance with thepresent invention;

FIG. 8 is a flow diagram of a process of an access via an RACH inaccordance with the present invention;

FIG. 9 shows a transmit power ramp up where the transmit power isincreased in-between each RACH slot;

FIG. 10 shows a transmit power ramp up where the transmit power isincreased during each RACH;

FIG. 11 shows a transmit power ramp up in a distributed channel;

FIG. 12 shows an RACH for a localized mode of frequency multiplexing;and

FIG. 13 shows an RACH for a distributed mode of frequency multiplexing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

When referred to hereafter, the terminology “WTRU” includes but is notlimited to a user equipment (UE), a mobile station, a fixed or mobilesubscriber unit, a pager, a cellular telephone, a personal digitalassistant (PDA), a computer, or any other type of user device capable ofoperating in a wireless environment. When referred to hereafter, theterminology “Node B” includes but is not limited to a base station, asite controller, an access point (AP), or any other type of interfacingdevice capable of operating in a wireless environment.

FIG. 1 shows a schematic flow of uplink data transmission in an OFDMsystem. A discrete Fourier transform (DFT), (equivalently, fast Fouriertransform (FFT)), is performed on user data symbols to be transmitted bya DFT unit 101, (or FFT unit). The resulting data after DFT processingis mapped to a group of sub-carriers by a mapping unit 102. Thesub-carrier mapping may be either localized sub-carrier mapping or adistributed sub-carrier mapping. Next, an inverse fast Fourier transform(IFFT), (equivalently inverse DFT), is performed by an IFFT unit 103 (orinverse DFT unit) on the sub-carrier mapped data. A cyclic prefix (CP)is then attached by a CP unit 104 before transmission of the sub-carriermapped data.

FIG. 7 shows a wireless communication system 700 in accordance with thepresent invention. The system 700 comprises a Node B 730 incommunication with at least one WTRU 710, 720 in a cell. A WTRU 710, 720includes a processor 711, a multiplexer (MUX) 712, and a transmitter713. FIG. 7 shows components of the WTRU 710 separately, but somecomponents may be implemented by more or less components. The WTRU 710sends an RACH transmission to the Node B 730 over an access channel 701,(i.e., an RACH), for an initial access. After detecting the RACHtransmission from the WTRU 710, the Node B 730 returns an ACK over anACK channel 702.

It may take several RACH transmissions before the Node B 730 detects theRACH transmission from the WTRU 710. The WTRU initially sets a transmitpower level of the RACH transmission to a predetermined level andincreases the transmit power level for the subsequent RACHtransmissions. Alternatively, the WTRU may ramp up the transmit powerlevel of the RACH transmission while transmitting the RACH transmission,which will be explained in detail hereinafter.

A set of subcarriers is assigned for an RACH. Subcarriers assigned forthe RACH may be either “localized” or “distributed” in a frequency band.FIG. 2 shows a localized mapping where a block of consecutivesubcarriers 201 are assigned for an RACH. FIG. 3 shows a distributedmapping where a plurality of distributed subcarriers 301 across thefrequency band are assigned for an RACH.

The random access procedure includes sending a signature sequence andperforming time and or frequency multiplexing RACH and datatransmissions. A plurality of orthogonal signature sequences are definedand the WTRU 710 sends one of the signature sequences via an RACH.

The RACH transmissions include a transmit power ramp up in transmissionof the signature sequence. As explained hereinbefore, since there is nocode to detect, the ramp up can be faster. In a conventional OFDMsystem, no code search is performed. However, in accordance with thepresent invention, the RACH is searchable over subcarriers for thepresence of a pattern.

FIG. 4 shows an OFDM subframe structure for the RACH in accordance withthe present invention. A 0.5 ms OFDM subframe includes a plurality ofshort blocks (SBs) and a plurality of long blocks (LBs). Each of the SBsand LBs are separated by a CP. The RACH includes control data includedin a LB and/or uplink reference symbols included in an SB. The uplinkreference symbols include both channel estimation and channel qualityindication (CQI) measurements. The uplink reference symbols areorthogonal each other and are either: (1) multiplexed auto (differentsets of sub-carriers); (2) time multiplexed; (3) code multiplexed(different shifts of a constant amplitude zero auto correlation (CAZAC)sequence).

In accordance with one embodiment of the present invention, RACHtransmissions and data transmissions are time multiplexed. All terminalsthat are communicating have been time synchronized to the Node B. In avariation of the first embodiment, there is a slot for RACH transmissionto happen between data frames, or multiple data frames as shown in FIG.5. The multiplexing is accomplished either in software operating on ageneral purpose processor, or using dedicated logic circuit to performthe multiplexing. A dedicated logic circuit would simply include aswitch 501 that switches between a data stream 502 and RACH information503, taking a fixed number of bits/symbols from each one in order. AnRACH access may occur during the random access slots. Alternatively, theRACH access may occur every few data frames.

Multiple RACHs may be defined as different sets of subcarriers. FIG. 6shows three (3) RACHs, (RACH1, RACH2, and RACH3), as an example, eachwith a unique set of subcarriers. For RACH transmission, one of thedefined RACHs may be chosen by a WTRU randomly, or may be assigned to aWTRU based on predetermined criteria. One way to assign a different RACHto a different user is using a serial number of a WTRU. Alternatively,any other criterion, (such as user ID), that is unique to each user maybe used for the RACH assignment. As an example, if the slots areassigned based on the last digit of a user specific number, and if thenumbers are random to begin with, users may be grouped into ten groups(one group for each slot 0, 1, 2, . . . , 9).

On a downlink ACK channel, which is used to let a user know that a WTRUis recognized by a Node B after the WTRU makes an RACH transmission, theNode B may use a set of sub-carriers (i.e., a subchannel) associatedwith the RACH channel carrier assignments. This enables a user trying toaccess the Node B to see an ACK message intended for that user.

Additionally, each WTRU may select a random phase of a ConstantAmplitude Zero Auto Correlation (CAZAC) sequence to further randomizeand avoid collisions between accessing users. In such case, the totalnumber of RACHs is increased by the following formula:

Total Number of RACH=N _(subchannels) ×N _(CAZAC−Random phases).

FIG. 8 is a flow diagram of a process of an access via an RACH inaccordance with the present invention. A WTRU 710 selects an RACH amonga plurality of pre-defined RACHs (step 801). The WTRU 710 preferablyselects a phase for the overlaid CAZAC sequence (step 803). The WTRU 710sets up a transmit (Tx) power for the RACH while making an RACHtransmission (step 805). The RACH transmit power may be ramped up whilethe RACH transmission is being transmitted as shown in FIG. 9.Alternatively, the transmit power may be stepped up each subsequent RACHtransmission as shown in FIG. 10. A Node B processor 732 recognizes theWTRU's attempt on the RACH (step 807). The Node B responds on anassociated ACK channel (step 809). The Node B 730 also sends TAinformation to the WTRU (step 811). The WTRU 710 stops increasing theRACH transmit power once the WTRU 710 receives an ACK from the Node-B730 (step 813). The WTRU 710 then adjusts TA (step 815) and sends anRACH message (step 817).

A Node B has to implement a detector that will search all RACHs duringRACH intervals for WTRUs. The RACH slot must be larger than the tripdelay from the Node B, and the slot must also have a CP at the end so asnot to interfere with data frames. Associated ACK channels have to bepaired with the RACH and CAZAC phase detected. Additional informationmay be coded on the phase of the RACH access.

FIG. 11 shows a signal response for the distributed transmission optionwhich is essentially the same as the localized option. In the localizedtransmission, all subcarriers in the subcarrier block for the RACH aretransmitted and the power of all of them is ramped up, or stepped up. Inthe distributed option shown in FIG. 11, the distributed subcarriers aretransmitted and the power of those distributed subcarriers is ramped up,or stepped up. If the first RACH transmission is not successfullyreceived by a Node B, the WTRU sends more than one RACH transmissionswhile increasing transmit power on the RACH as shown in FIGS. 9-11.Where the RACH access takes more than one RACH slot, the transmit poweron the subsequent RACH slot may be the transmit power at the last RACHtransmission, or it can be lower or higher than the previous one. Thepower ramp-up may initially start at a level that is low enough not tocause interference with other cells.

In accordance with another embodiment of the present invention, some ofthe subcarriers are used for the RACH while others are used for dataand/or control data (i.e., frequency multiplexing). The RACH subcarriersmay be localized or distributed as shown in FIGS. 12 and 13. Since allWTRUs are synchronized to a Node B, the WTRUs know the timing and havecarrier synchronization. The RACH frame may be a special frame andsimply use the same frame structure as the data and control fields.

In this embodiment, power ramp-up operates essentially the same as thetime multiplexed embodiment described hereinbefore. A WTRU synchronizesto the Node B and starts ramping up on the access channel. The RACH canhave multiple channels with different subcarrier assignments or CAZACsequences can form multiple RACH channels. Different phases of the CAZACsequence can be used to increase the number of RACHs.

The Node-B detects the phase of the RACH sequence and the phase can beused to increase the number of RACHs. In other words, since the Node Bis capable of detecting different phases of the CAZAC sequence, andCAZAC sequences with different phases are orthogonal to each other(therefore do not interfere with each other), multiple RACH attempts canbe made by multiple WTRUs at the same time. This in effect increases thenumber of available RACHs.

It is important to note that both the time and frequency multiplexedembodiments described above can allow the Node B to transmit an ACK on ashared channel with an indication of the recognized RACH attempt, whichis readable by all WTRUs.

There are several considerations that are specific to the MIMOapplication. Multiple MIMO options are possible including spatialmultiplexing, space-frequency block coding (SFBC), space time blockcoding (STBC), beam forming, and other combinations of these options.When making an attempt to access a Node B, the WTRU should be detectedquickly so that the WTRU does not ramp-up too high and causeinterference to other WTRUs on the system. Therefore, the WTRU makes theRACH attempt using the MIMO scheme with the highest redundancy among theones possible. Here, the MIMO scheme with the highest redundancy refersto the MIMO scheme that includes the highest level of diversity gain andtherefore is most robust to different channel conditions. Diversity MIMOtechniques, (such as STBC or SFBC), take advantage of the diversity gainand enable signaling that is resistant to fading and other channelimpairments.

Different MIMO options that may be considered in accordance with thepresent invention are as follows:

The first option is called a basic STBC option, in which the WTRU makesan RACH access attempt using an open loop STBC. Using an open loop STBChas an advantage that the signal benefits from diversity gain withoutrequiring expensive receivers at the Node B.

The second option is called a dominant mode option, in which the WTRUhas downlink channel estimation and may formulate the eigen modes of thechannels. The WTRU determines the best mode of the channel by analyzingthe signal from the Node B and use the same mode to transmit back. Thishas an advantage that an effective best beam forming solution isimplemented that would increase signal quality at the Node B and makedetection performance higher.

Another option is an optimum precoding option. When precoding beamforming is available, the WTRU may select the best precoding option tomake an RACH attempt. If a codebook approach is used, the WTRU choosesthe precoder that is either most robust for maximum diversity, or hasthe highest gain. Optimum precoding option is similar to the dominantmode option in that the WTRU applies a precoding matrix to the transmitsignal where the precoding matrix is selected from a codebook, and suchthat the precoding matrix is best suited for the prevailing channelconditions.

After a successful RACH attempt, adaptive modulation coding (AMC) andlink adaptation may start. Which option is implemented depends on thechannel conditions, the cell type (i.e. hotspot, macro cell, micro cell,etc), and the capabilities of the WTRU.

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the preferred embodiments or in various combinations with orwithout other features and elements of the present invention. Themethods or flow charts provided in the present invention may beimplemented in a computer program, software, or firmware tangiblyembodied in a computer-readable storage medium for execution by ageneral purpose computer or a processor. Examples of computer-readablestorage mediums include a read only memory (ROM), a random access memory(RAM), a register, cache memory, semiconductor memory devices, magneticmedia such as internal hard disks and removable disks, magneto-opticalmedia, and optical media such as CD-ROM disks, and digital versatiledisks (DVDs).

Suitable processors include, by way of example, a general purposeprocessor, a special purpose processor, a conventional processor, adigital signal processor (DSP), a plurality of microprocessors, one ormore microprocessors in association with a DSP core, a controller, amicrocontroller, Application Specific Integrated Circuits (ASICs), FieldProgrammable Gate Arrays (FPGAs) circuits, any other type of integratedcircuit (IC), and/or a state machine.

A processor in association with software may be used to implement aradio frequency transceiver for use in a wireless transmit receive unit(WTRU), user equipment (UE), terminal, base station, radio networkcontroller (RNC), or any host computer. The WTRU may be used inconjunction with modules, implemented in hardware and/or software, suchas a camera, a video camera module, a videophone, a speakerphone, avibration device, a speaker, a microphone, a television transceiver, ahands free headset, a keyboard, a Bluetooth® module, a frequencymodulated (FM) radio unit, a liquid crystal display (LCD) display unit,an organic light-emitting diode (OLED) display unit, a digital musicplayer, a media player, a video game player module, an Internet browser,and/or any wireless local area network (WLAN) module.

1. A wireless transmit/receive unit (WTRU) for accessing a wirelesscommunication system, the WTRU comprising: a receiver configured toreceive random access channel (RACH) assignment information indicatingsubchannels to be used for a RACH transmission; a transmitter configuredto transmit a combined phase of a first constant amplitude zeroautocorrelation (CAZAC) sequence and the cyclic prefix over a RACH to aNodeB; the receiver further configured to receive an indication over ashared channel that the phase of the first CAZAC sequence was receivedat the NodeB, wherein the indication includes a timing advance (TA); andthe transmitter further configured to transmit a signal using adaptivemodulation and coding, the signal including access message data andreference symbols with a transmission timing based on the TA to NodeB;wherein the reference symbols are derived from a second CAZAC sequence.2. The WTRU of claim 1 wherein the combined phase of the first CAZACsequence and the cyclic prefix is transmitted over a plurality ofsub-carriers.
 3. The WTRU of claim 1 wherein the wireless communicationsystem uses orthogonal frequency division multiplexing (OFDM).
 4. TheWTRU of claim 1 wherein the wireless communication system usesorthogonal frequency division multiplexing (OFDM) multiple inputmultiple output (MIMO).
 5. The WTRU of claim 1 wherein the combinedphase of the first CAZAC sequence and the cyclic prefix is transmittedin a slot.
 6. The WTRU of claim 1 wherein the received combined phase ofthe first CAZAC sequence and the cyclic prefix is frequency multiplexedwith data or control signals from other WTRUs.
 7. The WTRU of claim 1wherein information is conveyed by the transmitted first CAZAC sequenceand cyclic prefix.
 8. The WTRU of claim 1 wherein the receiver isfurther configured to receive RACH assignment information for aplurality of RACHs; wherein the first CAZAC sequence is randomlyselected and the RACH is randomly selected from the plurality of RACHs.9. A method for accessing a wireless communications system comprising,the method comprising: receiving, at a wireless transmit/receive unit(WTRU), random access channel (RACH) assignment information indicatingsubchannels to be used to transmit a RACH transmission; transmitting thecombined phase of a first constant amplitude zero autocorrelation(CAZAC) sequence and a cyclic prefix over the RACH to a NodeB;receiving, at the WTRU, an indication over a shared channel that thephase of the first CAZAC sequence was received by the NodeB, wherein theindication includes a timing advance (TA); and transmitting a signalusing adaptive modulation and coding, the signal including accessmessage data and reference symbols with a transmission timing based onthe TA; wherein the reference symbols are derived from a second CAZACsequence.
 10. The method of claim 9 wherein the combined phase of thefirst CAZAC sequence and the cyclic prefix is transmitted over aplurality of sub-carriers.
 11. The method of claim 9 wherein thewireless communication system uses orthogonal frequency divisionmultiplexing (OFDM).
 12. The method of claim 9 wherein the wirelesscommunication system uses orthogonal frequency division multiplexing(OFDM) multiple input multiple output (MIMO).
 13. The method of claim 9wherein the combined phase of the first CAZAC sequence and the cyclicprefix is transmitted in a slot.
 14. The method of claim 9 wherein thecombined phase of the first CAZAC sequence and the cyclic prefix isfrequency multiplexed with data or control signals from other WTRUs. 15.The method of claim 9 wherein information is conveyed by the transmittedfirst CAZAC sequence and cyclic prefix.
 16. The method of claim 9wherein the RACH assignment information include RACH assignmentinformation for a plurality of RACHs; wherein the first CAZAC sequenceis randomly selection and the RACH is randomly selected from theplurality of RACHs.
 17. A wireless transmit/receive unit (WTRU) foraccessing a wireless communication system, the WTRU comprising: areceiver configured to receive random access channel (RACH) assignmentinformation indicating subchannels to be used to transmit a combinedphase of a first constant amplitude zero autocorrelation (CAZAC)sequence and a first cyclic prefix; a transmitter configured to transmitthe combined phase of the first CAZAC sequence and the first cyclicprefix over a RACH to a NodeB at a first power level; the transmitterfurther configured to transmit a second combined phase of the CAZACsequence and a second cyclic prefix over a RACH to the NodeB at a secondpower level that is greater than the first power level on a conditionthat an indication that the transmitted combined phase of the firstCAZAC sequence and the first cyclic prefix was not received by theNodeB; the receiver further configured to receive an indication over ashared channel that the transmitted combined phase of the second CAZACsequence was received at the NodeB, wherein the indication includes atiming advance (TA); and the transmitter further configured to transmita signal using adaptive modulation and coding, the signal includingaccess message data and reference symbols with a transmission timingbased on the TA to NodeB; wherein the reference symbols are derived froma third CAZAC sequence.